The present invention relates broadly to apparatus for detecting and locating hidden variations in the density of a mass having a smooth outer surface.
The invention has particular application in the detection and location of separations or air pockets hidden within truck tire casings during inspection to determine fitness for re-treading.
As is well known, thousands of heavy duty trucks, notably tractor-trailers, travel the highways of the country every day, each equipped with a plurality of heavy duty tires. A typical tractor-trailer may, for example, have as many as 18 heavy duty tires engaging the roadway at the same time. Such tires involve high original cost and it is common practice, when the tread is worn, to inspect the casing to determine whether or not it is in sufficiently good shape to warrant re-treading. Thousands of re-treaded truck tires roll along the highways every day.
In inspecting a tire casing to determine if it is fit for re-treading, it is difficult but important to determine whether there are any defects hidden from visual detection. In the case of bias-ply tires, the most common hidden defect is that of a separation between the plies of the fabric, or a separation between the plies and the rubber. Such separations create pockets which trap air. Such air pockets disqualify the tire as a candidate for re-treading. For if the pocket or separation is not detected and the tire is retreaded and used, the pocketed air will become heated to high temperatures during use, and will expand and place severe stresses on the casing, sufficient in many cases to cause blow-outs. This, of course, is to be avoided, not only because the re-treading will have been wasted but also because of the danger of life and property which may result from the blow-out.
To detect such separations hidden within the tire casing, it is known in the prior art to have a workman tap the casing repeatedly with a hammer and to listen for changes in the tonal quality of the sound produced. Such method is, however, not dependable in that some areas of the tire casing may easily be missed without the workman knowing that he has missed them. Moreover, his tapping may not be uniform in intensity. Also, the workman may not be able to detect differences in the sounds produced because of the environmental or ambient noise level in the area in which he is working.
It is also known in the prior art to use ultrasonic methods for detecting separations in tire casings. However, in such prior art method, the ultrasonic frequencies are applied to one surface of the casing (either the exterior or interior surface) and detected at the other. Detection of separations is made difficult by interference caused by the presence of stones, nails, glass and the like which are lodged in the tire tread. In contrast to such prior art ultrasonic system, in the system of the present invention an impact is applied to the interior surface of the casing and the sound waves produced are detected at the same surface.